The fourth spacewalk of the mission to repair and update the Hubble Space Telescope is under way, with astronauts working to bring back to life a key instrument that has been down for the last five years.

The Imaging Spectrograph basically is NASA's major black hole hunter. It acts as a sort of outer space prism, separating light from the universe into its component colors, giving scientists a "wavelength fingerprint" of any object. That data can offer clues to an object's temperature, chemical composition and motion.

The spectrograph is designed to pick up the light and color signature of black holes. It also helps astronomers map the motion of gas affected by a black hole's gravitational pull.

The spectrograph, which was installed on Hubble during a 1997 shuttle service mission, suffered a power supply failure in 2004 and has since been in safe mode, which means it's off but protected, NASA said.

Massimino and Good are scheduled to replace a low-voltage power supply board that contains the failed power converter.

The astronauts also are prepared today to install a stainless steel blanket on the outside of the telescope. The blanket, which should offer thermal protection for equipment bays, will replace multi-layer insulation that has started to fall apart.

In Saturday's spacewalk, astronauts John Grunsfeld and Andrew Feustel worked to repair Hubble's Advanced Camera for Surveys -- the instrument behind some of the telescope's most impressive images of deep space. It stopped working a little more than two years ago.

Wearing thick gloves and orbiting 354 miles above the Earth, Grunsfeld and Feustel removed electronics cards and cut through metal yesterday to repair the three cameras in the instrument. However, NASA announced this morning that one of the cameras did not come back to life after Saturday's spacewalk. "The repair of the Advanced Camera for Surveys' high resolution [camera] was always considered a long shot," NASA said on its site. "It is likely down for good."

NASA said it still considers the overall repair mission of the Advanced Camera for Surveys a success because two of the three cameras are functioning.

Monday's spacewalk, which will be the fifth and the last of the mission, will focus on installing a battery module containing three batteries. On Tuesday, the seven-astronaut crew will deploy the telescope. They're scheduled to return to Earth on Friday.

This story, "NASA astronauts repairing Hubble's black hole hunter" was originally published by
Computerworld.